Make Event RefCountedGarbageCollected and implement trace() methods to the Event hierarcy

Source/core/events/PopStateEvent.h

 /*
  * Copyright (C) 2009 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 35 matching lines @@
     SerializedScriptValue* serializedState() const { return m_serializedState.get(); }
     void setSerializedState(PassRefPtr<SerializedScriptValue> state)
     {
         ASSERT(!m_serializedState);
         m_serializedState = state;
     }
     History* history() const { return m_history.get(); }
 
     virtual const AtomicString& interfaceName() const OVERRIDE;
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     PopStateEvent();
     PopStateEvent(const AtomicString&, const PopStateEventInit&);
     explicit PopStateEvent(PassRefPtr<SerializedScriptValue>, PassRefPtr<History>);
 
     RefPtr<SerializedScriptValue> m_serializedState;
     RefPtr<History> m_history;
 };
 
 } // namespace WebCore
 
 #endif // PopStateEvent_h